Hi bro, for batteries, it is only used to start the car and to
maintain the memory of the electronic devices. Thus, when it is
low/dead, you will not be able to start the car.
When your engine is running, all the electricity comes from the
engine, thus, even if you remove your battery when your engine is
running, nothing will happen.
But if let's say you leave your headlight on thru the night, and
the battery is flat the next morning. All you have to do is to get
someone to jumpstart your car for u, or if it's a manual car, to
push start it. Once the engine is started, the battery will begin
to charge again and you can continue using the battery.
But if the battery is spoilt, even if you manage to jump/push start
it, once u off the engine u will have problems starting it
again.
The lifespan of a battery is about 1 to 1 and a half years, but
most are warranty-ed for only 1 year.
For maintanence-free batteries, there is usually a window where u
can see into the battery, and you can guage the state of the
battery by the colour u see inside. Otherwise, u can't really tell
except by using devices from workshops.
Hope this helps. 
